Studies on improvement of acoustic bragg imagery

Abstract

Acoustic Bragg Diffraction Imaging (ABI) enables non-invasive visualization of object details opaque to light. It renders real-time imaging, preserves phase information, and is simple in implementation. Even though ABI has been in use for some years now, it requires further investigation and development to reach its full potential in non-destructive testing and medical diagnosis. The aim of this thesis is to present the results of studies on the factors responsible for the loss of image details in ABI and to suggest techniques for improving the information content of Bragg diffraction imagery.

Studies on the characteristics of Bragg images are presented. An optimum alignment procedure for setting up a Bragg diffraction imaging system is discussed. An interferometric study of degradations contributed by various components of the system is carried out. For the correction of aberrations, a holographic compensator is realized, and the improvement achieved in the impulse response is reported. The relevance of using holographic optics in ABI is highlighted. Restoration of aspect ratio by a holographic cylindrical lens is suggested, and the results obtained are described.

Bragg images also suffer from coherent noise such as speckle and ringing arising from the spatial and temporal coherence of the acoustical and optical radiations. Suppression of the coherent noise is carried out by frequency modulation of the sound source. Although this reduces speckle and ringing, it introduces blur in the image as a result of frequency modulation. This blur is studied in detail, and restoration has been carried out using a complex filter in an optical spatial filtering configuration.

For improvement in dynamic range, the technique of multifrequency Bragg imaging is suggested. Images of the same object obtained at different frequencies have been added using a colour additive viewer, and the resultant image is found to offer better discrimination of image details.

In the concluding chapter, a Bragg imaging system evolved from the various studies mentioned above is presented. Also included in this chapter are suggestions for further studies.